1. Field of the Invention
This invention relates to electronic music synthesis and in particular is concerned with the assignment of a limited number of tone generators to actuated keyboard switches.
2. Description of the Prior Art
The current trend to use microelectronic implemented tone generators for keyboard operated musical instruments has led to the development of systems which achieve a measure of economy by using a plurality of tone generators which is less in number than the number of keyboard switches in the keyboard array of keyswitches. An assignment logic is implemented to assign the members of the available tone generators to keyswitches as they are depressed to their actuated keyswitch states. An inevitable resource availaility problem arises when all the tone generators have been assigned to keyswitches and an additional keyswitch is actuated.
One of the earliest assignment systems was disclosed in U.S. Pat. No. 2,577,493 entitled "Electronic Musical Instrument." In the disclosed system a number of analog oscillators are assigned by means of a multicontact set of keyswitch contacts to actuated keyswitches. The assignment action also introduces a preselected capacitor so that the assigned oscillator generates a signal whose fundamental frequency corresponds to the assigned actuated keyswitch. The assignment system disclosed in the referenced patent does not teach any means to cope with the situation in which all the tone generators have been assigned and an additional keyswitch is actuated.
Because there are ten fingers available to the musician, it would seem that ten tone generators would suffice for any keyboard. Modern keyboard electronic instruments have incorporated a tone effect that demands more than one tone generator to be assigned for a single finger. This tone effect is given the generic name of "sustain." Unfortunately this term has led to some confusion because the present terminology is to refer to an ADSR (attack/decay/sustain/release) time envelope modulation function. More properly the older term of "sustain" should now be called "long release". With a long release, it is a fairly easy matter to release a keyswitch and actuator another keyswitch with a single finger while the first tone generator is still operative and is automatically being reduced in tone volume by its ADSR envelope modulation function.
Several tone generator assignment logic systems have been disclosed which attempt to solve the availability problem. The inherent drawback with systems for assigning tone generators is that each of these systems has an assignment logic which is not ideal in the sense that one can readily postulate realistic musical playing techniques in which the assignment logic yields a result which may be contrary to the effect expected or desired by the musician.
A tone generator assignment system is disclosed in U.S. Pat. No. 3,610,806 entitled "Adaptive Sustain System For Digital Electronic Organ." In the disclosed system, a plurality of tone generators are provided each of which can be assigned to any keyswitch on any of the instrument's arrays of keyswitches. The long release mode is normally used on only one keyboard at a time. The adaptive sustain mode is entered only, and automatically, on the basis of internal information indicating (a) which tone generators have been committed to the keyboard demanding long release, (b) which of these tone generators that have been committed to a long release is producing a waveform envelope that has undergone the longest release time, and (c) which, if any, of the tone generators are idle and therefore available for assignment. However, as soon as all the tone generators have been assigned, the system automatically enters the adaptive sustain mode in which any tone generator assigned to a note associated with a key on the manual having a long release mode, and which generator is supplying the waveform that had had the longest duration of a release phase, is switched immediately from a long release to a relatively shorter release time. This action is purported to expedite the availability of a tone generator for assignment of the next note in a rapidly keyed sequence of notes.
One of the principal defects in assignment logic occurs when a long release time effort has been selected and the musician attempts to play a glissando by "dragging" a finger along the keyboard to quickly actuate a sequence of keyswitches. This quickly leads to a total assignment of all the available tone generators and then blank tones, or missing notes in the glissando sequence, occur until some of the older assigned tone generators completely finish their release phase of their ADSR envelope modulation function. At this time they can be reassigned but usually not before the blank tones have been encountered.